1. Field of the Invention
The subject invention relates generally to a patch antenna. Specifically, the subject invention relates to a patch antenna for receiving circularly-polarized radio frequency signals from a satellite.
2. Description of the Related Art
Satellite Digital Audio Radio Service (SDARS) providers use satellites to broadcast RF signals, particularly circularly polarized RF signals, back to receiving antennas on Earth. The elevation angle between a satellite and an antenna is variable depending on the location of the satellite and the location of the antenna. Within the continental United States, this elevation angle may be as low as 20° from the horizon. Accordingly, specifications of the SDARS providers require a relatively high gain at elevation angles as low as 20° from the horizon.
SDARS reception is primarily desired in vehicles. SDARS compliant antennas are frequently bulky, obtuse-looking devices mounted on a roof of a vehicle. SDARS compliant patch antennas typically have a square-shaped radiating element with sides about equal to ½ of the effective wavelength of the SDARS RF signal. When the radiating element is disposed on a window of the vehicle, this large “footprint” often obstructs the view of the driver. Therefore, these patch antennas are not typically disposed on the windows of the vehicle.
However, even when these patch antennas are disposed on the windows of the vehicle, certain parts of the vehicle, such as a roof, may block RF signals and prevent the RF signals from reaching the antenna at certain elevation angles. Even if the roof does not block the RF signals, the roof may mitigate the RF signals, which may cause the RF signal to degrade to an unacceptable quality. When this happens, the antenna is unable to receive the RF signals at those elevation angles and the antenna is unable to maintain its intrinsic radiation pattern characteristic. Thus, antenna performance is severely affected by the roof obstructing reception of the RF signals, especially for elevation angles below 30 degrees. In order to overcome this, a radiation beam tilting technique can be used to compensate for signal mitigation caused by the vehicle body. Since antennas capable of receiving RF signals in SDARS frequency bands are typically physically smaller than those antennas receiving signals in lower frequency bands, it becomes challenging to tilt the antenna radiation main beam from the normal direction to the antenna plane, which is substantially parallel to the glass where the antenna is mounted.
Various patch antennas for receiving RF signals are well known in the art. Examples of such antennas are disclosed in the U.S. Pat. No. 4,887,089 (the '089 patent) to Shibata et al. and U.S. Pat. No. 6,252,553 (the '553 patent) to Soloman.
The '089 patent discloses a patch antenna having a radiating element. A first feed line and a second feed line are electrically connected to the radiating element at a first and second feed port, respectively. A switching mechanism connects a signal to either the first feed line or the second feed line. A horizontally polarized (i.e., linearly polarized) radiation beam is generated by the patch antenna in a higher order mode. However, the patch antenna of the '089 patent does not generate a circularly polarized radiation beam and is therefore of little value in the reception of circularly polarized RF signals broadcast from satellites.
The '553 patent also discloses a patch antenna having a radiating element. The antenna includes a plurality of feed lines electrically connected to the radiating element at a plurality of feed ports. The antenna also includes at least one phase shift circuit to shift a base signal and produce at least one phase-shifted electromagnetic signal. A circularly polarized radiating beam is generated by the patch antenna in both a fundamental mode and a higher order mode. The patch antenna of the '553 patent does not generate the circularly polarized radiation beam solely in a higher order mode. As such, the radiating element of the patch antenna of the '553 patent defines a large “footprint”.
There remains an opportunity to introduce a patch antenna that aids in the reception of a circularly polarized RF signal from a satellite at a low elevation, especially when the patch antenna is disposed on an angled pane of glass, such as the window of a vehicle. There also remains an opportunity to introduce a patch antenna which significantly reduces the required “footprint” of the antenna's radiating element when compared to other prior art patch antennas. There further remains an opportunity to introduce a patch antenna that can overcome interference caused by a roof of the vehicle.